[LARTC] Interface not marked down when links are down

[LARTC] Interface not marked down when links are down

[SPJ]

Hi,

I am not sure if this problem has already being
posted, but I haven't found any solution. Excuse me if
it's a repost but would appreciate any pointers. 
I have newly configured Multipath default route. I
have 1  local connection and 4 internet connections. I
am using kernel 2.6.7 and have applied Julian's
relevent patches. 
When the next hop is down, julian's patch takes care
of the links changing the route to only the links
which it finds are up and forwarding traffic to those
links. But if the problem is anywhere in between,
beyond the next hop, the links are not marked
identified as down (I down see the change in multipath
default route) and requests still go via them and
hence no connectivity, even if some other links are
up.
Does Julian's patch takes care of route only if next
hop is down. Any solution to this problem?

Thanks,
SPJ

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 
_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/

Re: [LARTC] Interface not marked down when links are down

[Rene Gallati]

SPJ wrote:
> I am not sure if this problem has already being
> posted, but I haven't found any solution. Excuse me if
> it's a repost but would appreciate any pointers. 
> I have newly configured Multipath default route. I
> have 1  local connection and 4 internet connections. I
> am using kernel 2.6.7 and have applied Julian's
> relevent patches. 
> When the next hop is down, julian's patch takes care
> of the links changing the route to only the links
> which it finds are up and forwarding traffic to those
> links. But if the problem is anywhere in between,
> beyond the next hop, the links are not marked
> identified as down (I down see the change in multipath
> default route) and requests still go via them and
> hence no connectivity, even if some other links are
> up.
> Does Julian's patch takes care of route only if next
> hop is down. Any solution to this problem?

You are looking at the wrong layer. If the next hop is unreachable, 
which is a layer 2 thing, the system notices this and acts accordingly. 
If however a link further away is down, there is no such mechanism on 
layer 2 and your system cannot react to it (with standard means).

This is quite "psychic" if you want. Why should your system magically 
know there is a problem 4 hops upstream ? It only sees the links it is 
directly attached. The packets are accepted on one link and this is 
where the responsibility of your multihomed system ends. It neither does 
nor should track packets over the full path from source to destination.

There is a solution however: For that kind of thing you need to look 
into routing software (BGP, OSPF) which has this capability. If you 
don't control the upstream machines, you will need to get a bgp-peering 
with your upstream provider to be able to react to outages. If your 
mentioned 4 connections are standard home-adsl/cable connections, this 
might be difficult however.

Hope that helps.
-- 

C U

      - -- ---- ----- -----/\/  René Gallati  \/\---- ----- --- -- -

_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/